This is Mazon Monday post #35. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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For this post, we are highlighting some of the pectens found in the Mazon Creek fossil deposit. Aviculopecten mazonensis is one of the most commonly known, but there are three others of note Dunbarella striata, Euchondria pellucida, and Palaeolima retifera. They are mostly found in Pit 11, Braceville, and other parts of the Essex biota. But, they were found in Morris at the old Chowder Flatts locality.
Aviculopecten mazonensis is in the Creature Corner book. The article first appeared in the November 1988 issue of the newsletter. Here is the text of the article by Andy Hay and a illustration drawn by Don Auler.
Aviculopecten mazonensis is the most common of the several pectinoids found in the Essex biota of the Francis Creek Shale, Carbondale Formation. A. mazonensis was the first figured and described by Worthen in 1890. The genus Aviculopecten can be found in strata of the Silurian to the Permian Period.
Our Creature has the familiar pecten outline, with a straight hinge line that is almost equal in width to the length of the shell (valve). lts dimensions are (29x 30 mm) about an inch in length and width in the larger specimens. Deeply notched auricles "ears" are near the hinge end. The valves are not symmetrical and are not equilateral in contour. The right and left valves are not equally convex. Numerous costae (ribs) on the valves are decorated with distinct fine lines. Auricles are also decorated with numerous costae. The costae on larger specimens exceed 50 in number. Aviculopecten lacked spines.
Assuming that A. mazonensis' life style to be similar to living pecten having the same shell configuration, it would be attached to the sea floor with its hinge line vertical; valves opened in the direction of sea currents to filter out passing food particles. This upright stance was maintained with the aid of byssussess (secreted filaments akin to silk) that held it to the sea bottom. To escape predators, valves would be "clapped" together, forcing water from the mantle. This jet-like stream broke it free of its moorings and away from danger.
Our Creature, Avicuiopecten mazonensis is classed: Phylum Mollusce; Class Bivalvia; Order isodonta; Superfamily Pectinacea; Family Aviculopectenidae; Genus Aviculopecten.
Next, let's have a look at the descriptions in the Mazon Creek Fossil Fauna book.
Avicuiopecten mazonensis
Aviculopecten mazonensis is the most common of the pecte noids among Essex Fauna bivalves. The hinge line is almost equal in width to the valve and its auricles (ears) are prominent, dis- similar, and deeply notched. The valves are not symmetrical nor equally convex. A. mazonensis has numerous fine costae (ribs) on the valves and auricles. The growth rings are weakly developed.
Modern pectens are common and colorful. They have rows of tiny eyes along the mantle edge which make them unique among bivalves. Normally, a pecten rests on the sea floor, but when disturbed, it can swim by rapidly clapping its two shell valves, creating a water jet for propulsion. A membranous fold beneath the shell (the muscular mantle) acts as a valve directingflow of the ejected water, and to a degree, maintains the direction of movement.
Dunbarella striata
Dunbarella striata is a pectenoid with a nearly circular shell. It has auricles (ears) that are similar in width, but only one side has a pronounced notch. The hinge line is nearly straight.Valves are almost equal in length and width and are decorated with numerous fine costae (ribs). One valve is slightly flatter than the other. Dunbarella is uncommon to rare, and occurs in the Essex Fauna.
Euchondria pellucida
Euchondria pellucida is the rarest ofthe Mazon Creek pectenoids. Smaller than Aviculopecten mazonensis though comparable in overall outline, it is surmised that they shared a similar lifestyle. E. pellucida likely lived exclusively in brackish water. What sepa- rates this species from the other scallops is its shell ornamentation. The left valve (top in life position) has thin radiating lines; crossing these are equally fine, regularly placed, concentric lines producing a checkerboard appearance. The right valve (bottom) has almost no ornamentation, displaying only weakly marked growth lines.
Palaeolima retifera
Palaeolima retifera has an inflated and obliquely oval shell. The valvesare similar and attached along a narrow hinge. It has prominent and pointed beaks, with distinct radial costae (ribs).
P retifera was a filter-feeder that lived on the surface of muddy bottoms and is most oftenfoundin an open death pose. Adorned with colorful tentacles, modern marine examples are sedentary, living in nests built from shell fragments held together by their byssi (silk-like filaments.) If needed, they can swim for short distances similar to other scallops by clapping their valves together.
Here are a couple fossil pictures. The first in Dunbarella from Braceville and the second is Aviculopecten from Chowder Flatts.
